1. Field of the Invention
This invention relates to the testing of materials and to the measurement of deflections in materials and more particularly, but not exclusively, to the measurement of such deflections and deformations which occur at high speed, acceleration and frequency during structural testing.
2. Discussion of Prior Art
The measurement of deflections and deformations forms one of the key areas of research and development in the field of materials testing. With the increased availability of new and high performance materials such as carbon fibre composites industry has been investing in the development of useful applications for such materials. Investigation into the engineering potential of these materials has been a major source of investment in many industries including the aerospace industry. The driving force behind the structural engineering research and development being conducted within the aerospace industry is to develop a lighter, stronger and more cost effective alternative to traditional metallic alloy structures and components.
Modern computational software analysis allows the simulated testing of new materials over a wide ranges of `in service` load scenarios such as static, fatigue, creep, and dynamic loading conditions.
State of the art materials testing methods used for the determination of deflections in mechanical structure include the use of electromechanical transducers to physically measure movement in discrete directions and optical methods that rely on phenomena such as Moire fringe Interferometry.
In the case of impact and fatigue testing, software techniques are being more extensively used in order to help overcome practical difficulties associated with the particular requirements of such tests. For example, in testing the effects of birdstrikes on aircraft structures, the speed, acceleration and frequency of deflection of the structure and therefore the information capture rates required make the use of techniques such as projection Moire fringe Interferometry both technically complicated and expensive.
A proposed alternative method for measuring high speed, acceleration and frequency deflections in structures utilises the out of plane movement of a projected laser spot focused onto a panel under test. The reflected spot is focused onto a CCD element and the movement of the spot across the CCD element is calibrated such that it is proportional to the deflection of the panel under test. The method provides a digital measurement system by virtue of the fact that a CCD element is essentially a linear array of discrete charge coupled devices arranged to receive light energy inputs. The light input readings recorded by each element of the CCD take a finite time to be converted into recordable information and subsequently moved to a register address for future recall and analysis, consequently limitations with this method become prevalent as the speed, acceleration and frequency of deflections to be measured increase.
Any testing method that requires the recording of high speed events relies on the ability of the system to respond at a capture rate fast enough to ensure that all relevant deflection data has been recorded. For very high speed applications such as birdstrike analysis the laser spot and CCD element method does not have a sufficiently fast capture rate and so a higher capture rate method is required.
High speed photography using both film and video media is also used for the recording and analysis of deflections in materials under test. The capture rates of such systems are typically in the order of 1000 frames per second but for the purposes of ballistic testing e.g. birdstrike tests on aircraft structure, the capture rate would need to be in the order of 6000 frames per second. These capture rates are possible with new film techniques but require levels of event, film and lighting synchronisation that make their use both technically very difficult and expensive.